1. Field of the Invention
The present invention relates to an optical waveguide, and an optical waveguide device provided with electrodes for deflecting, switching, or modulating laser beams guided into the optical waveguide by the electo-optic effect. The invention relates especially to a light beam deflection device for a laser printer, digital copying machine, and facsimile, an optical switch and optical waveguide modulation device for an optical transmission or optical computer, and an optical waveguide device applicable generally to the opto-electronics including a pickup for an optical disk.
2. Description of the Related Art
As a laser beam deflection device used in a laser beam printer, digital copying machine, and facsimile, there is a device constructed by the so-called rotary polygon mirror to deflect laser beams from a gas laser or a semiconductor laser and an f.theta. lens that condenses the laser beams reflected by the rotary polygon mirror in the state of the linear uniform motion on an image-formation plane of an electrophotographic photoreceptor or the like, which has been generally used. This type of the light beam deflection device using the polygon mirror has a problem in durability, because the polygon mirror is rotated in a high speed by a motor, and a problem that the high speed rotation generates noises and the light scanning speed is limited by the number of revolutions of the motor.
On the other hand, as a solid state laser beam deflection device, there is a light beam deflection device that utilizes the acousto-optic effect, and above all the optical waveguide device is noticed with expectations. This optical waveguide device is examined as a laser beam scanning device to solve the disadvantage of the laser beam scanning device using the polygon mirror, as to whether it can be applied to a printer and the like. The light beam deflection device of this optical waveguide type possesses an optical waveguide composed of LiNbO.sub.3 and ZnO, and a coupling unit to guide laser beams into this optical waveguide; and further contains a comb-type electrode to excite a surface acoustic wave that deflects the light beams in the optical waveguide on the basis of the acousto-optic effect, and a unit to output the deflected light beams from inside the optical waveguide. In addition to this, a thin film lens and the like are attached to the device as needed. However, the light beam deflection device employing the acousto-optic effect generally has the problem of the upper limit of laser beam deflection speed due to the deflection speed limit, which leads to a limit of the application to an image formation device such as a laser printer, digital copying machine, facsimile, and the like.
In contrast to this, there is a prism-type light beam deflection device employing an oxide ferroelectric material capable of displaying the electo-optic effect of which modulation speed is higher than the acousto-optic effect, which is described in, for example, `Optical Electronics, 4th ed. by A. Yariv (New York, Rinehart and Winston, 1991), page 336.about.339`. This type of device includes a bulk device using ceramics or single crystals, and it has a large size and requires a considerably high drive voltage, so that a practical deflection angle cannot have been obtained. And, `Q. Chen, et al., J. Lightwave Tech. vol. 12 (1994) page 14011` (reference 1) and the Japanese Published Unexamined Patent Application No. Hei 1-248141 disclose a prism-type domain inversion light beam deflection device or a prism-type electrode light beam deflection device having prisms arrayed in cascade by using a LiNbO.sub.3 single crystal wafer which a Ti diffusion-type optical waveguide and a proton exchange-type optical waveguide are made of. However, the thickness of the LiNbO.sub.3 single crystal wafer, about 0.5 mm, is needed for the electrode gap. Accordingly, the drive voltage is still high, and the foregoing reference 1 discloses that the deflection angle of 0.2 is barely obtained with the drive voltage 600 V applied, which poses a problem that this type of device does not produce a practical deflection angle.
The inventors of this invention have invented a prism-type light beam deflection device to solve the problem of the drive voltage by using a thin film optical waveguide which is provided with an oxide optical waveguide displaying the electo-optic effect disposed on a conductive substrate, a light source to emit light beams into this optical waveguide, and an electrode for deflecting the light beams in the optical waveguide by means of the electo-optic effect. This is disclosed in the Japanese Published Unexamined Patent Application No. Hei 9-5797.
However, the distribution of the electromagnetic field by the laser beams propagating through the optical waveguide permeates into the substrate. The absorption coefficient of a substrate having a practical resistivity is high, and in most cases, the permeating component is strongly absorbed by the carrier inside the conductive substrate. Therefore, the propagation loss in the thin film optical waveguide amounts to some 10 dB/cm due to the absorption, in addition to the loss due to the scattering of the optical waveguide itself, which is insufficient for a practical use.
Further, when the upper electrode employs an oxide, the transparency is not sufficient, thereby the propagation loss is increased, and the micro fabrication is not easy as compared to a metal, which is also a problem.
Generally, in the device having a coplanar electrode configuration, a cladding layer of SiO.sub.2 is interpolated between a metal electrode on an optical waveguide and the optical waveguide, which prevents an electromagnetic field from permeating into the metal electrode, thus avoiding the absorption of the propagating light. However, the relative dielectric constant of an oxide optical waveguide material displaying the electo-optic effect amounts to several tens to several thousands, which is extremely large compared to the relative dielectric constant 3.9 of SiO.sub.2. Further, the foregoing thin film optical waveguide structure on the conductive substrate forms a series capacitor as the equivalent circuit, and therefore, the effective voltage applied to the thin film optical waveguide goes down to less than several percent of the voltage applied, which results in a great increase of the drive voltage.